The present invention relates to signal analysis, and more particularly to a wideband signal analyzer for analyzing next generation cellular phone signals.
A conventional frequency domain signal analyzer is shown in FIG. 1. A frequency converter 12 converts a signal under test down to an intermediate frequency (IF), and a bandpass filter 14 eliminates undesired frequencies outside the passband. A local oscillator 16 and a mixer 18 further convert the signal into a second IF that is converted by an analog-to-digital converter (ADC) 20 into digital time domain data. A digital signal processor (DSP) 22 produces frequency domain data from the time domain data by calculation, such as a fast Fourier Transform (FFT). The frequency domain digital data is stored in a memory 24 and may be displayed as a waveform on a display 26.
Due to the Nyquist limit, one-half the sampling frequency for the ADC 20 determines an upper limit of a frequency bandwidth for the signal analyzer. For wider bandwidths a higher sampling frequency is required. However a higher sampling frequency results in a decrease in the number of bits provided by the ADC 20 and an increase in the distortion characteristics of any IF amplifier prior to the ADC. The result is a decrease in dynamic range. Frequency domain analysis requires relatively high resolution, such as 12-14 bits, which requires a lower sampling frequency, so there is a conflict between the resolution (dynamic range) and bandwidth requirements.
U.S. Pat. No. 6,340,883 discloses one solution to the bandwidth versus dynamic range problem by separating the signal under test into I and Q signals using an analog demodulator at the input and then converting the I and Q signals separately into digital data so that it decreases the required sampling frequency of the ADCs for a given dynamic range and bandwidth by one-half. This makes it easier to deal with high speed and wideband signal processing. U.S. Pat. No. 6,356,067 further discloses a technology for effectively measuring a wideband signal by using wide and narrow band processing paths appropriately.
The dynamic range for current products, such as the Tektronix WCA380 Wireless Communication Analyzer which uses the I/Q split technique, is about 55 dB for a wide span of 30 MHz. The Agilent 89600 Vector Signal Analyzer alternatively uses a fast ADC to provide approximately the same dynamic range over the same wide span. These current products cannot provide the same dynamic range over the wider span necessary for the measurement of next generation cell phones. Although U.S. Pat. No. 6,340,883 makes the required sampling frequency one-half due to the I/Q split of the input signal, there is a limit to further widening of the bandwidth.
What is desired is to provide an architecture for wideband signal processing of next generation cell phones that provides a wider span independent of the ADC sampling frequency.